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A new window for the study of exotic atomic nuclei.

29.01.2004

On Friday the 30th, during the XLII international winter meeting on nuclear physics at Bormio, the first results will be announced of Finuda experiment (Nuclear Physics at Daphne), settled in Frascati at Infn National Laboratories.

Planned and made operating by a group of about forty physicists from Universities and Infn Sites of Bari, Brescia, Frascati, Pavia, Torino and Trieste, Finuda is devoted to the study of hypernuclei: nuclei composed by three different kinds of particles rather then two (protons and neutrons) as in ordinary nuclei.

The first stage of the experiment started on October the15th and the data obtained up to now promise to be the most relevant in the study of hypernuclei since their discovery occurred in 1953 thanks to the Polish physicists Marian Danysz e Jerzy Pnieswski. “The study of hypernuclei opens a favoured window to understand some aspects of the strong force and the weak force which, with electromagnetic and gravitational forces, complete the whole of the four fundamental forces in nature” says Sergio Bertolucci, director of the National Laboratories of Frascati.

Hypernuclei are the result of an alteration of a normal atomic nucleus, obtained hitting an ordinary nucleus through a particle named K meson. In the collision between K mesons and nucleus, one of the particles that composes the nucleus, that is to say a proton or a neutron, is substituted by another particle named lambda. While protons and neutrons are composed by such quarks named up and down, the lambda particle contains a third type of quark named strange. This one is provided with a greater mass rather than the one of up quark and down and it is considered it was plentiful in the very first instants of universe life.

New information about the structure of atomic nuclei can be obtained studying the way the lambda particle localizes inside nucleus. Protons and neutrons are in fact joined inside nucleus by the so-called strong force, but they go through the effects of Pauli exclusion principle, which prevents identical particles, such as two protons or two neutrons, from locating inside nucleus in the same energy level (energy levels depend on how near to the centre of nucleus they are). But in a hypernucleus there is only one lambda particle, so it is free from the effects of exclusion principle and when it localizes inside nucleus, it is subject only to the strong nuclear force.

A second kind of interesting information comes from the fact that the lambda particle is unstable: it tends to decay in a very short time, retransforming itself in a proton or a neutron. This is due to the weak interaction: one of the four fundamental forces, the same which origins the natural radioactivity.

Nevertheless, if the lambda particle is embedded in the nucleus, the presence of protons and neutrons and the Pauli principle prevent it from decaying in the usual ways. Therefore, it must interact with the other components of the nucleus before retransforming in a proton or a neutron. It’s just studying these mechanisms that Finuda can clear up some aspects of the weak interaction till now unexplored.

“Finuda has been planned to study both formation of hypernuclei and their decaying and this makes it an innovative experiment”, says Tullio Bressani, national spokesman of Finuda experiment.

The hypernuclei studied by Finuda are produced thanks to the accelerator named Dafne at Frascati National Laboratories. Dafne permits collisions between electrons and positrons creating, as a final product of a series of transformations, the K mesons.

“Despite the great interest of hypernuclei study, it proceeded slowly until some years ago, because of the great difficulties occurred in producing a satisfactory number of them” explains Tullio Bressani, “foreign laboratories, in the United States and above all in Japan, have recently made a notable experimental effort in this research field, but even more significant has been the one made in Italy at Frascati National Laboratories of Infn.

The only initial phase of data recording, which will continue for about three months, will allow to produce many hundreds of thousands of hypernuclei, more or less the same quantity obtained and observed in the first fifty years since their discovery”. The success of the experiment is also a success of Italian research: “Finuda is attracting to our country, and in particular to the laboratories of Frascati, the best researchers in the world in the field of hypernuclei physics”, concludes Sergio Bertolucci.

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